If you are still unable to login after confirming your username and password are correct you'll want to try clearing the cache and cookies on your browser. Follow the instructions below to clear this information from your Safari browser.

Safari 8
From the Safari menu, select Clear History and Website Data....
Select the desired time range, and then click Clear History.
Go to Safari > Quit Safari or press Command-Q to exit the browser completely.

Safari 7 and below
From the Safari menu, select Reset Safari....
Select the items you want to reset, and then click Reset. As of Safari 5.1, Remove all website data includes both cookies and cache.
Go to Safari > Quit Safari or press Command-Q to exit the browser completely.